This invention relates to seismic geophysical surveying. Specifically, it is directed to a system in which each seismic trace is recorded at a group location and without the need for multiconductor cables or equivalent to transmit received seismic signals from the individual groups forming the seismic spread, to a central recording point. In this particular system, small, portable recording units are placed at the seismometer group locations. Each such portable recording unit including a source of electric power. Since such sources use conventional batteries and must necessarily be severely limited in available energy in order to maintain small size, hence mobility of the system, it is extremely desirable that each such unit be turned on as nearly as possible coincident with the time at which recordings are to be made, and turned off promptly after the recording. This makes maximum use of the limited battery energy available at the various group recorders. Accordingly, seismic group recorders have been arranged for turning-on from a remote point, referred to as the control point, ordinarily by means of a radio link, or its equivalent. This requires that all seismic group recorders must be within communication range of the control point. Ordinarily, as mentioned above, this link is radio, employing the licensed transmission bands available. This particular invention constitutes an improvement in the communications employed in such a system, so that maximum range for turning on with accuracy the selected group recorders and transmitting necessary identifying information is secured for taking each record.